In hearing aid circuits, there is a problem with sound coupling along external feedback paths through the air. The external feedback generates annoying whistles and audio distortion. The external auditory canal, for example, is not sealed by the hearing aid. There is an external feedback path that couples sound produced by a hearing aid receiver through the auditory canal to a hearing aid microphone.
In some hearing aid designs, a portion of the hearing aid is positioned in the ear canal and includes a vent that contributes to the gain of the external feedback path. In other hearing aid designs, the sound from the receiver couples via a narrow tube into the auditory canal, and there is a feedback path in the space around the narrow tube. Frequently, jaw motion can change the shape of the ear canal, opening up additional air paths that can contribute to the gain of the external feedback path. When a sound reflecting object such as a telephone earpiece is brought near the hearing aid, sound reflections can also contribute to feedback path gain. The characteristics of the external feedback path are variable and real time correction is desired. Various feedback cancellation circuits are known, as shown in FIG. 1 for example. However these feedback cancellation circuits typically have problems distinguishing between sounds from the environment, such as musical notes, and actual feedback.
A hearing aid circuit is needed that can distinguish feedback from environmental sounds, and that can improve cancellation of feedback without unduly distorting environmental sounds.